A quantum dot (QD) is a tiny speck of matter so small that it is effectively concentrated into a single point (i.e., zero dimensional). As a result, the particles inside it that carry electricity (electrons and holes) are trapped or constrained and have well-defined energy levels according to the laws of quantum theory. Typically, QDs are crystals a few nanometers wide, so they are a few dozen atoms across and contain anything from perhaps a hundred to a few thousand atoms. QDs are made from a semiconductor such as silicon and although they're crystals, they behave more like individual atoms.
QDs may be controlled precisely to have wide applications in the real world. As a general background, if an atom is given energy, it can be excited (i.e., boost an electron inside it to a higher energy level). When the electron returns to a lower level, the atom emits a photon of light with the same energy that the atom originally absorbed. The color (i.e., wavelength and frequency) of light an atom emits depends on what the atom is because of the way their energy levels are arranged. Generally speaking, different atoms give out different colors of light. This is because the energy levels in atoms have set values (i.e., they are quantized).
QDs do the same, as they also have quantized energy levels. However, QDs made from the same material will give out different colors of light depending on how big they are. A small QD has a bigger band gap, which, roughly speaking, is the minimum energy it takes to free electrons so they'll carry electricity through a material, so it takes more energy to excite it. Because the frequency of emitted light is proportional to the energy, smaller QDs with higher energy produce higher frequencies and lower wavelengths. Larger QDs have more spaced energy levels and thus produce lower frequencies and higher wavelengths.
As a result, the biggest QDs produce the highest wavelengths (and shortest frequencies), while the smallest QDs make shorter wavelengths (and higher frequencies). This generally means that big QDs produce red light and small QDs produce blue, while medium-sized QDs produce green light (and other colors too).
In recent time, the fabrication of self-assembled QDs has been intensively investigated because of their potential for novel optoelectronic device applications, such as lasers, solar cell and light-emitting diodes. Indeed, the optoelectronic properties of QDs are strongly associated with their physical properties such as size, composition, strain and shape which determine the confinement potential of the electrons and holes. Thus, the growth mechanisms for fabricating active quantum nanostructures are becoming important.
Among various growth techniques, the most common approach is Stranski-Krastanov (SK) growth mode that is based on self-assembled mechanism and typically used in lattice mismatched systems, such as InAs/GaAs systems. In the SK growth, a thin film of semiconductor is grown on a semiconductor substrate, generating a lattice mismatch at the interface of the two materials. During the epitaxial growth, the interlayer mismatch strain is partially relaxed and three-dimensional structures are then formed. However, the morphology and composition of QDs will be significantly changed during the deposition of a capping layer, which make it difficult to achieve the designed properties. Furthermore, this technique was not available in lattice-matched system such as GaAs/AlGaAs systems due to the absence of strain.
An alternative and promising technique for fabricating strain-free GaAs/AlGaAs QDs is droplet epitaxy (DE) growth mode which is first demonstrated by Koguchi and Ishige in 1993. In comparison with SK technique, DE technique can be used in both lattice-mismatched and lattice-matched systems and thus have high design flexibility. In the case of GaAs QDs, numerous metallic Ga droplets are first formed on the substrate in the absence of As4 flux. Droplets are subsequently crystallized through exposure to an As4 flux for the formation of GaAs QDs. Ga droplets are usually formed at low temperature (˜300° C.) in order to maintain their original morphology. However, such low temperature often causes degradation of crystalline and optical qualities during the deposition of AlGaAs capping layer. Moreover, this low-temperature surrounding also strongly influences the formation of GaAs materials while incorporating Carbon dopants.
It has been demonstrated that the intensity of As4 flux and crystallization temperature will determine the final morphology. For example, generally GaAs QDs are not Carbon-doped and are formed at low temperature (around 100° C.-200° C.) with an As4 fluxes of 10−4-10−5 Torr at the crystallization step. Single, double or multiple quantum rings (QRs) are generated under the As4 flux of 10−6-10−7 Torr at the growth temperature between 200° C.-450° C. at the crystallization step. The holed nanostructures are grown at higher crystallization temperature (T=450° C.-620° C.).
A series of previous studies have been demonstrated that the experimental conditions can determine the final structures. However, some drawbacks are still existed. In case of GaAs QDs formation, Ga droplets are formed at low temperature in order to maintain their original morphology. Such low temperature growth process often causes degradation of crystalline and optical qualities during the deposition of AlGaAs capping layer. Therefore, more works are necessary for the fabrication of high-quality QDs.
In addition, recombination between confined electrons and photo-excited holes bound to acceptor impurities has a number of advantages for the study of novel physical phenomena in semiconductor nanostructures. Since bound-hole energy is very well defined, photoluminescence (PL) is a direct measurement of the energy spectrum of the electronic states, and localization of the hole relaxes k-conservation rules such that the entire electronic density of states can be investigated. This technique was used very successfully to probe the physics of two-dimensional (2D) electron system, leading to optical investigation of Landau levels, Shubnikov-de Haas oscillations, the fractional quantum Hall effect and Wigner crystallization. However, it is believed that there is no equivalent work involving zero-dimensional (quantum-dot) structures.
Therefore, there is a need for a new zero-dimensional electron device consists of highly-uniform Carbon-doped GaAs and/or a highly-uniform GaAs and the methods of making the same. This new zero-dimensional electron device has broad applications in the semiconductor field including QD lasers, solar cell, light-emitting diodes, single photon sources for quantum cryptography, quantum bits, and quantum logical elements.